Decide on the semantics of origin-visit status(es)

For #2310 (closed), we want to add a new object type: origin visits status. Each visit would have multiple statuses, but all of these objects are immutable:

• origin_visit stores the initial state of the visit
• origin_visit_statuses are created at each visit state change (e.g when a loader starts or updates a visit).

There are two meanings we can give to origin_visit_status:

1. a snapshot of the state of the loading at a given date (ie. all fields are present)
2. a "patch" over the previous status update

Cons of 1:

• More duplication, so possible way more space needed if we start using the metadata field of visits (we currently don't)

Cons of 2:

• harder to implement, especially for the pg storage
• less efficient as you may need to read multiple visits (although not that much)
• state is not inherently consistent, eg if we're missing an older update (eg. while replaying); although we could add a "pointer" on the previous update to detect when it's missing, and temporarily return an error

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Migrated from T2346 (view on Phabricator)